1. Technical Field of the Invention
The present invention relates generally to signal transmission systems constructed of physical cables, and more particularly to a method for protection switching in case of detecting faults in such signal transmission systems.
2. Description of the Related Art
Signal faults in signal transmission systems that use a physical cable for signal transmission include reduced signal strength that results in signal degradation and complete loss of signal (e.g., break in the cable). In digital transmissions systems requiring high reliability, rapid signal fault detection is often used to enable the traffic to be re-routed so that there is little discernible loss of signal to the user when a fault occurs. In a conventional signal transmission system using signal transmission cables, one method of signal fault detection is to monitor and detect a signal fault at a receiving end of a first transmission signal cable and if a fault is detected a fault notification message is sent back to the originating (transmitting) end via a different (e.g., second) cable so that any fault in the first transmission cable will not keep the fault notification message from arriving at the originating end. Once a fault notification message is received by the originating end via the second cable, the signal transmission system re-routes the signal traffic from the cable on which a fault has been detected to another transmission cable. This type of fault detection protocol is in many circumstances slow to provide the desired level of user indiscernible signal reception when a fault occurs along a cable transmission signal path.
For example, the greater the length of the transmission path the longer time it will take a signal to propagate from one point to another on the transmission path, and thus the longer the time delay between the time when a signal fault occurs and the time when the signal is re-routed and recovered. This delay is of particular concern in, for example, transoceanic and transcontinental transmission because of the great lengths a signal (both the original signal and error notification message signals) must travel before it reaches its destination. The longer this distance and related signal propagation delay, the more likely that an end user will sense that the transmission has been dropped when a fault occurs in a transoceanic or transcontinental transmission cable path (e.g., a degradation or break in the transmission signal). A specific example of this conventional fault detection and restoration method is provided below.
FIG. 1 illustrates one signal transmission system that would use the fault detection method described above. In this example, the signal transmission system may be a telephone communication system having a first node 10 which includes a transmitter connected to cable 13. The transmitter in node 10 sends voice signal transmission on cable 13 to node 12, node 12 having a receiver. In this case, node 10 may be referred to as the originating or upstream cable station and node 12 may be referred to as the receiving or downstream cable station. The voice signals in this case may be transmitted on either of two frequency bands, X and Y bands 14 which accommodate a respective signal and operate in a unidirectional manner from node 10 to node 12. If cable 13 is a fiber optic cable (single fiber line) that includes double band Erbium doped fiber amplifier (EDFA), then the X band may be for example a conventional amplification band (C band) which has a typical range of amplification wavelength band between 1525 nm to about 1570 nm, and the Y band may be for example a long wavelength amplification band (L band) which has a typical range of amplification wavelength band between 1560 nm to about 1610 nm. The signal transmission system also includes a second cable 15 over which voice and/or data signals may be transmitted from node 12 to node 10. Cable 15 may likewise be a fiber optic cable that includes dual band EDFAs with X and Y bands 16. The X 18 on cable 13 illustrates the location of a signal fault, for example an unacceptable signal degradation or a break in a signal being transmitted from node 10 to node 12.
The conventional method for fault detection in the transmission system of FIG. 1 will now be explained with reference to FIG. 1 and the process flow diagram of FIG. 2. First, at step 20 a signal (e.g., voice or data signal) is transmitted on a first cable A, cable 13, from the upstream originating node, node 10, to a downstream receiving node, node 12. This signal may travel on either X band, Y band, or both X and Y bands 14. Next in step 22, the signal is received at the downstream receiving node 12. At step 24, the receiver of node 12 connected to cable 13 monitors the incoming signal for a fault. If a fault is not detected in the received signal at node 12, the receiver continues to monitor the received signal at step 22. However, if an error is detected at step 24, for example a break occurs at X 18 (see FIG. 1), a transmitter of node 12 (downstream node) connected to a second cable, cable B (cable 15), sends a fault notification message on the second cable B (15) to a receiver of the upstream originating node 10. Finally, at step 28, the transmission system activates re-routing of the data signal traffic transmitted from the upstream originating node 10 to the downstream receiving node 12 on another cable (or fiber) so as to restore error free data traffic from node 10 to node 12.
Using the conventional error detection method, the time it takes the transmission system to restore error free signals includes (1) signal propagation delay time, (2) failure detection time, (3) hold-off time, (4) switching time, and (5) frame synchronization time. The signal propagation delay time includes a first propagation delay time T2 along cable 13 from X 18 to node 12, which represents the time it takes for a lost or degraded signal that occurs at fault X 18 to reach node 12, and a propagation delay time T3 from node 12 to node 10, which represents the propagation time it takes the error notification to travel from node 12 to node 10. These time delays can be considerable in transoceanic and transcontinental signal transmission where the cable lengths can span thousands of miles or kilometers. This translates into typical propagation delays (T2+T3) on the order of, for example, 50-100 milliseconds (ms) for a fiber optic transoceanic cable which may be too long in some systems and leads to total delays on the order of 300 ms from the time an error occurs until an error free signal is restored, such as a telephone voice signal transmission. This restoration time can result in providing a system user unacceptable service, for example, if a fault occurs along a long length telephone signal cable the cumulative signal recovery time my result in the telephone user concluding that the telephone call has been dropped. Therefore, there is a need for a fault detection method in signal transmission systems which reduces the delay associated with protection switching protocol and reduces the time span between the time a fault occurs and the time the transmission system restores error free signal transmission from one node to another node.
The present invention provides a method of detecting the loss or degradation of a transmitted signal in a signal transmission system. The method for detecting and correcting signal transmission faults in a signal transmission system includes bi-directional signaling on the same fiber, cable, or signal line. A fault in a transmitted signal from a first upstream node to a second downstream node is determined by the loss or degradation of a signal simultaneously transmitted from the second downstream node to the first upstream node transmitted in the opposite direction on the same fiber, cable, signal line. As a result, the propagation delay of the transmitted signals necessary for protection switching protocol and fault protection, and the amount of time to restore fault free signal transmission between one node and another node in a signal transmission systems is reduced, thereby increasing user satisfaction with minimal signal transmission interruption in the case of a transmission fault.
In one feature of a preferred embodiment of the present invention, the method uses bi-directional signals on a single optical fiber connected between an originating upstream cable station and a receiving downstream cable station. A first voice, data or video signal is transmitted from the originating upstream cable station to the receiving downstream cable station using a first signal wavelength while simultaneously a second signal is transmitted from the receiving downstream cable station to the upstream originating cable station using a second signal wavelength complementary to the first voice or data signal. If a fault occurs on the optical fiber between the two cable stations the fault is detected by the upstream cable station as a result of monitoring the second signal sent by the downstream cable station. The only propagation delay in detecting the fault is the time it takes the second signal to propagate from the location of the fault to the upstream originating cable station. If a degradation or break in the second signal occurs, the upstream originating cable station determines that the first voice or data signal it originally sent to the downstream cable station is experiencing the same fault and thus re-routes the signal so as to be received error free by the downstream cable station.
In another feature of the present invention, a transmission line fault is detected prior to transmission of a desired signal. In this alternative, a signal used for fault detection is transmitted from the downstream node to an upstream node before the desired transmission signal is sent from the upstream node to the downstream node. Thus, the transmission line may be checked for faults prior to transmitting the desired transmission signal. If a fault is detected on the transmission line prior to beginning transmission of the desired signal, an alternative traffic route may be predetermined before transmission of the desired signal on the faulty transmission line so that the transmission of the desired signal begins on a fault free transmission line.